If you’re showing two maps, where a different variable is displayed on each, the convention seems to be to assign those two variables different color ramps. For example, you might make a map of income in New York City and give the income variable a red color ramp, and then you might make a map of Whites or Non-Hispanic Blacks in New York City and give those variables different color ramps, say green, or orange. But if you need to compare these variables and do a sort of visual overlay of them in your mind, I find that my brain focuses first on the change in color rather than on the change in concentration or color saturation of the variables. This ultimately makes the change in pattern between the two variables harder to discern.
If you go against convention and use the same color ramp on both maps, I think this implies less that the variables are the same (which is the fear) and emphasizes more the patterns that emerge between the two – meaning, you will begin to see the patterns and potential relationships between the two variables immediately, not after your brain adjusts from the color change. Having two different color ramps might actually contribute to missing small differences and patterns that may be important.
Giving different variables the same color ramp emphasizes the importance of a good, clear title since that is what your map readers will rely on to know which variable they are looking at.
The two sets of maps below show examples of maps that both use the same color ramps for different variables and map that use different color ramps for different variables. Which set of maps helps to make the differences/similarities/patterns between the maps within each set stand out more clearly?